Soil salinity is one of the most significant abiotic stresses for plant agriculture and therefore it would be useful to identify and isolate stress tolerance genes for the practical goal of genetically improving the salt tolerance of crop plants.
Two other major abiotic stresses, drought and cold, are intimately linked with salt stress. Many genes that are regulated by salt stress are also responsive to drought or cold stress (Zhu, 1997), therefore these genes are particularly interesting for genetically improving of stress tolerance.
The molecular mechanisms by which plants respond to salt stress are starting to be elucidated (Hasegawa et al. 2000a). The sodium transporters at the vacuole and plasma membrane, identified as the products of the Arabidopsis NHX1 (Gaxiola et al. 1992, Apse et al. 1999) and SOS1 (Shi et al. 2000) genes respectively, have been described as important determinants of salt tolerance.
For the goal of genetically improving stress tolerance in plants it is important to use stress tolerance genes that when introduced can immediately confer stress tolerance. The action of these genes cannot be dependent on other pathway-related events or other components that are necessary for the molecular mechanism of stress tolerance. One can identify important stress factors in the stressed organism, but the question remains whether these genes will also contribute in enhancing stress tolerance in a heterologous host when isolated and transfected herein.
Sugar Beet (Genes) and Stress
It is known that Beta vulgaris L. (Chenopodaceae, sugar beet), is rather a stress resistant plant when compared to other plants e.g. Arabidopsis thaliana. Sugar beets are relatively spoken rather stress resistant to salt- and drought. The genes that are responsible for the ability of sugar beet to grow in more difficult conditions are started to be elucidated. A first indication that a gene under investigation might be involved in the induction of resistance, is the increase of its expression under stress conditions. As an example, Matthias et al. (1996) showed that salt stress induces the increased expression of V-type H+ATPase in mature sugar beet leaves. Also betaine, an osmoprotectant, is accumulated by many beet plants in response to salinity and drought. Furthermore in sugar beet, the expression of Choline monooxygenase, which catalyzes the committing step in the synthesis of glycine betaine of sugar beet, is also induced by osmotic stress in Chenopodaceae. The mRNA levels in leaves increased 3- to 7-fold at 400 mM salt and returned to uninduced levels (Russel, 1998). As mentioned above there are several alternative pathways to respond to stress situations and therefore many different genes are probably involved in stress responses.